1. Field of the Invention
The present invention relates to a reflection-type liquid crystal displaying device that can be applied to various displaying apparatuses
2. Description of the Related Arts
Liquid crystal displaying devices consume less electric power than TV cathode-ray tubes since liquid crystal displaying devices employ a non-light-emitting method in which external light is modulated. Also liquid crystal displaying devices exhibit an excellent property that they are thin and light, suitable for application to flat panel displaying apparatuses. Therefore, by utilizing these features, liquid crystal displaying devices are now used for displaying apparatuses such as watches, electronic calculators, computer terminals, notebook-type computers, and word processors. Further, liquid crystal displaying devices are now used for displaying in television receivers instead of cathode ray tubes and, at present, they are used for various application fields.
A well-known representative display mode for modulating the light intensity by utilizing liquid crystals in a liquid crystal displaying device is what is called a TN (Twisted Nematic) mode in which the orientation of the liquid crystal molecules in a liquid crystal cell is twisted by about 90.degree. as initial orientation. In the TN mode, liquid crystal cells are disposed between a pair of polarizing plates, and black-and-white display is realized by utilizing the optical properties of the liquid crystal cells (namely, the rotatory power exhibited when no voltage is applied and dissolved when a voltage is applied).
In liquid crystal displaying devices of a simple matrix driving type, an STN (Super Twisted Nematic) mode is widely adopted. The STN mode utilize a cell structure which is similar to that of the TN mode and in which the twisted angle of the liquid crystal molecules are set to be within the range from 180.degree. to 270.degree.. In the STN mode, a sharp change in the orientation state of the liquid crystal molecules in accordance with the increase in applied voltage is reflected to change in birefringence (double refraction) of the liquid crystal by setting the twisted angle of the liquid crystal to be more than 90.degree. and further optimizing the set angle of the polarizing direction of the polarizing plates, thereby realizing an electro-optical property having a sharp threshold value. Accordingly, the STN mode can provide an electro-optical property having a sharp threshold voltage and, hence, is suitable for liquid crystal displaying devices of a matrix driving type. However, the STN mode has a disadvantage that the background of the display turns yellowish green or dark navy-blue due to birefringence of the liquid crystal. For reducing the above disadvantage, a liquid crystal displaying device has been developed in which black-and-white display is provided by color correction. The color correction is performed by superposing an optical compensating panel or a phase difference plate formed of a polymer such as polycarbonate on the STN liquid crystal display panel.
In a liquid crystal color displaying device having such a structure, multi-color display or full-color display is realized by additive color mixture by providing, for example, three-dot (red, green, and blue) micro color filters of minute dimension for each display pixel within a liquid crystal cell. However, these display modes have a disadvantage that the view angle is narrow due to the use of a polarizing plate and also, in principle, the display will be extremely dark because half of the incident light is not utilized for the display. Therefore, these display modes could not be applied to reflection-type liquid crystal displaying devices.
Accordingly, for use which requires brightness, a reflection-type liquid crystal display mode that is supposed to have a good prospect is what is called a guest-host mode in which a pigment (a dichroic pigment) having two different light absorptivities with respect to longitudinal and lateral directions of the pigment molecule is added to liquid crystals without using a polarizing plate. These guest-host modes may be classified into a Heilmeier type which uses a polarizing plate (See G. H. Heilmeier et al. Appl. Phys. Lett., 13, pp. 91, 1968), a White/Taylor type which does not use a polarizing plate (phase transition type: See D. L. White et al. Appl. Phys. Lett., 25,683, 1974), a two-layer type (See T. Uchida, Proc. SID, 22, pp. 41, 1981), and others. The principle of operation in these types of display modes is the same. Namely, the principle of operation in the guest-host mode is that the orientation of a pigment molecule is controlled via the orientation of the liquid crystal molecules by applying a voltage so that the difference in light absorptivity with respect to the direction of the pigment molecule may be reflected to the display.
Another method for providing a color display in the guest-host mode involves utilizing a pigment which absorbs visible light of a certain wavelength. In addition to this, there is proposed in Tohru Koizumi et al. Proceedings of the SID Vol. 29/2, pp. 157, 1988 a reflection-type liquid crystal color displaying device in which guest-host cells utilizing a black pigment are combined with a micro color filter. This article discloses a method of manufacturing a reflecting plate for a reflection-type liquid crystal color displaying device, the reflecting plate having an optimal reflection property by a control of the irregularity (unevenness) of the reflecting surface. This method employs subjecting a glass substrate to surface roughening treatment with a polishing agent, varying the time for etching the substrate with hydrofluoric acid so as to control the irregularity of the substrate surface, and forming a thin Ag film on the irregular surface to complete the reflecting plate.
However, in the above method of fabricating a reflecting plate for a reflection-type liquid crystal color displaying device, the glass substrate surface is damaged by a polishing agent for forming the irregular portion, so that it is not possible to form an irregular portion having a uniform shape with good reproducibility. Therefore, it is extremely difficult to provide a uniform reflection property with good reproducibility by such a reflection-type liquid crystal displaying device obtained by using a reflecting plate fabricated to have an ununiform irregularity formed on the glass substrate.